Whoever of you sees something evil, let him change it with his hand; and if he is not able, then with his tongue; and if he is not able, then with his heart, and that is the weakest of faith

The Prophet (may Allah's peace and blessings be upon him) commands that evil deeds - they are everything prohibited by Allah and His Messenger - should be changed, according to one's ability. So, if one sees some evil deed, he should change it with his hand, if he has the ability to do so. If he is unable to do this, then he should change it with his tongue, by forbidding the one who commits it, demonstrating its harm to him, and guiding him to goodness instead of that evil.

If he is unable to fulfill this level, then he should change it with his heart, by disliking that evil deed and determining to change it if he has the ability. Changing evil deeds with the heart is the lowest level of faith in this regard.